In WCDMA many users share the same frequency band. The transmission of one user thereby becomes interference for all other users in the uplink (and downlink). Interference is a problem for any receiver as it makes it difficult for the receiver to receive the sent out signal or transmission correctly. Often, errors are introduced in the received signal as a result of interference.
In order to overcome problems caused by interference, interference cancellation, IC, is used. When the transmission of a user equipment has been detected, or even decoded, the so obtained signal can be used to re-generate a model signal (a replica signal) that resembles the effect of the original transmitted signal of the user equipment, e.g. at the antenna or at another point in the receiver chain. The creation of the signal always requires that the channel model is available, to capture the effect of the radio transmission from the user equipment to the radio base station. The model signal can then be subtracted from the received broadband signal. In case the model signal is accurate, then the effect of the user on the uplink is significantly reduced. Since this signal is only interference for the other users, the effect of the subtraction is that interference as seen by the other users is cancelled, hence the acronym interference cancellation.
Interference cancellation is not a new technology in CDMA systems, and it exists in several variants. Interference cancellation can be based on demodulated IQ samples. Another option is to base the cancellation on decoded symbols. The latter option has the advantage of a better performance since the coding gain is exploited to enhance the quality of the model signal. On the other hand the delay is increased, due to the time needed for the decoding step. An additional delay is an inevitable consequence of most practical IC variants, since a model signal needs to be created before cancellation can be performed. Exceptions exist, however mostly at the price of an impractical computational complexity.
Another distinction of IC algorithms is between soft and hard algorithms—here the algorithms based on soft values are generally preferred, e.g. since today's turbo-codecs provide this information.
IC is computationally complex which creates problems when implementing IC in different architectures. There is a need to balance pure IC performance gains against the computational complexity and delay associated with different IC architectures.
The mobile broadband uplink also referred to as Enhanced Uplink, denoted EUL, operates by scheduling users to exploit the available load headroom in the best possible way. Since the headroom varies due to the load caused by already scheduled users, this scheduling needs to be fast. The scheduling is affected by delays of several scheduling intervals though (denoted transmission time intervals, TTIs), from the time a grant is scheduled and transmitted to a terminal, until the load appears on the uplink air interface as an interference power. This means that in order to be able to schedule so that the air interface load is kept below the maximum load needed to maintain stability and coverage of the cell, the scheduler needs to predict the load that appears over the air interface. In case without interference cancelling receivers, this prediction can e.g. be performed using measured Signal-to-Interference-Ratio, SIR, values, or alternatively powers, together with beta factors that define the offset of the data power of a user equipment, to the power controlled control channel. This load prediction is associated with errors, but is deemed sufficient for non-IC receivers.
The load prediction problems become formidable when IC receivers are introduced. The load appearing over the air interface is normally significantly reduced with IC receivers, as compared to non-IC receivers. The load reduction obtained by the IC receivers varies with the number of interfering user equipments in the cell, i.e. currently being served by a radio base station. Further, the load reduction obtained by IC receivers also varies with the bit rates of the interfering user equipments in the cell.